Video Routing: A look at what's next
Feb 1, 2009 12:00 PM, By Neil Maycock
Figure 1. A 4 x 4 crosspoint router
Click to enlarge
As more and more program flows involve files rather than traditional video signals, a debate has emerged over how, when or if traditional crosspoint routers should be replaced by IP networks. After all, hardly a day goes by without another task being performed via the mother of all networks, the Internet.
Also, recent major sporting events, such as the UEFA Euro 2008 Football Championships and FIFA World Cup Football, have been covered with high-bandwidth IP-based contribution networks at specific locations. Is it time to adopt the same principle for studio acquisition and program playout? The first step is to examine the benefits and drawbacks of each.
Crosspoint routing
Router technology changes allow users to control systems without needing to manage the underlying mechanisms that route their signals. Shown here is a typical user interface from Pro-Bel’s training facility.
The traditional crosspoint router is a matrix of electronic switches that can connect combinations of inputs to outputs. Typically an input signal can be switched to multiple outputs, making the router distributive. Figure 1 above shows a representation of a crosspoint router with four inputs and four outputs. The crosses represent active switches connecting an input to an output.
For a clean video switch, the devices handling the video have to operate synchronously with the video, switching between sources in a nonvisible part of the video signal. This requires switches to be accurately timed to at least 40ms in the case of standard definition.
Another key requirement for the crosspoint router is to have 100 percent availability for any switch. With a separate electronic switch for every possible path through the router, it is inherent in the design.
However, this guaranteed availability comes at a price. Figure 2 below illustrates how the cost of a crosspoint router increases approximately as a square law (note, this also applies to the physical size), and Figure 3 on the next page shows how the percentage of actual bandwidth used at any one time in the router follows an inverse trend.
Network routing
So how does this compare with a network? A network consists of interconnected components or circuits, or in computing terms, it's a system that delivers a means of transferring data between devices. This is in essence what the crosspoint router does, so does a network offer us an alternative means of performing this task, and are there any benefits?
Figure 2. The cost of a crosspoint router increases approximately as a square law.
Click to enlarge
Almost certainly there are no network topologies that follow the same cost and bandwidth curves shown in Figures 2 and 3. Therefore, there is a potential saving to be made by better use of a system's available bandwidth.
In a crosspoint router, as a switch is provided for every possible route, the system bandwidth is related to the number of crosspoints, which is the number of inputs multiplied by the number of outputs. In a network router, the bandwidth only needs to be related to the number of inputs. So the cost of a network router increases in a linear fashion, as opposed to the square law of the crosspoint topology.
Media networks distribute information by switching packets of data. However, the packetization and switching of individual packets requires data processing at the network nodes, which introduces some form of delay within the network. What these delays might be and how they can be managed are key factors when considering the real-time distribution of video.
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